Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards. Non-CDMA systems include the AMPS and GSM systems.
When a mobile station first acquires a system, one or more available base stations or cells are identified by the mobile station. Typically a single cell is selected as the serving cell. When the mobile station is not actively communicating for a voice call or data session, it is said to be in idle mode. The mobile station monitors signals from the serving cell to determine if an event, such as an incoming call, would require the mobile station to leave idle mode. As the connnunication channel between the serving cell and the mobile station changes, the mobile station must periodically test the channel to determine if that cell is still suitable. For example, the communication channel will change as the mobile station moves between cells within a system or between one or more neighboring systems. Occasionally a new serving cell will be selected, a process known as cell reselection in W-CDMA, or idle mode handoff in IS-95 or cdmu2000.
During idle mode, a mobile station may enter a low-power state, or “sleep” state, to reduce power consumption. A mobile station may discontinue reception during sleep state to allow receive circuitry to be disabled. Periodically, the mobile station must come out of sleep state, or “wake up”, reacquire the serving cell, and monitor signals therefrom to determine if an incoming call is being directed to the mobile station. During this time, a cell reselection process may be performed to determine if cell reselection is necessary. It is desirable for the mobile station to go back to the sleep state as soon as possible if it is to remain in idle mode, in order to conserve power. If a cell reselection is performed, the mobile station will remain awake longer, in order to process the reselection.
As channel conditions change, for example, due to movement of the mobile station within the system, or changes in the interference experienced, the serving cell may need to be adapted accordingly. Two or more base stations may, at any given time, have comparable quality metrics. Slight changes in the channel environment may cause the ranking of these base stations to change. However, reselecting a cell with comparable quality to the current cell may yield only marginal improvement, or none at all. Since cell reselection increases the time required before returning to sleep, it is desirable to limit reselection that would yield marginal or no improvement.
Furthermore, variations in the relative quality of the channel may cause variations in the desire for cell reselection. In a relatively good channel environment, cell reselection may be less desirable, since the current serving cell provides ample communication quality. On the other hand, in a relatively poor channel environment, gains from cell reselection may improve communication performance. Thus, limiting cell reselection, adapted appropriately for changing channel quality conditions, can increase time spent in a low-power, or sleep, mode, thus reducing idle mode power and increasing standby time. There is therefore a need in the art for limiting cell reselection in response to a variable channel.